1. Field of the Invention
This invention relates to electromagnetic contactors controlled by a microcomputer, and more specifically to such a contactor having protection for loss of power to the microcomputer.
2. Background Information
U.S. Pat. No. 4,720,763 discloses an electromagnetic contactor which is controlled by a microcomputer. Such contactors are switch devices which are especially useful in motor starting, lighting control, and similar switching applications.
One of the functions of the microcomputer in the contactor of U.S. Pat. No. 4,720,763 is to control energization of the coil of the contactor electromagnet in a manner which assures rapid positive closure of the electrical contacts without damaging contact bounce. The microcomputer also provides overcurrent protection by monitoring current flowing through the contactor and tripping the contacts open if preset limits are exceeded.
The microcomputer and the remainder of the control circuit for the contactor are powered by a 60 Hz 120 volt source derived either from the line side of the contactor or an independent commercial source so that the control system remains operative even with the contacts of the contactor open. In the case of an overcurrent condition, the microcomputer prevents reclosure of the contacts for a specified period of time.
The microcomputer and the contactor of U.S. Pat. No. 4,720,763, are controlled by a custom integrated circuit chip similar to that disclosed in U.S. Pat. Nos. 4,626,831 and 4,674,035. This custom chip provides a reset signal to the microcomputer on power-up. The initialization process initiated in the microcomputer by the reset signal, includes checking the random access memory (RAM) for stored data indicating the status of the contactor. However, if power to the RAM has been interrupted, this data will be lost. Hence, if the contactor was in the trip state when power was lost, this status of the contactor would not be detected upon power up. Therefore, it is possible through momentary loss of power to lose the protection provided by the delay in restart. In fact, this might be done intentionally by an operator to override the delay and restart the motor or other component which receives power through the contactor.
Accordingly, there is a need for means for maintaining power to the memory of the microcomputer, at least during short-term power losses, so that the protection function of the contactor can be maintained.
There is a subordinate need for a reliable power source during the short-term loss of power.
There is also need for a simple, reliable, economical manner of transferring the microcomputer to the alternate power source upon loss of the conventional power source.